When coded data such as compressed bitmapped graphics is drawn, a method decoding data every time the data is redrawn makes processing late because the decoding takes time. Therefore, because the data decoded once is temporarily stored in a memory apparatus, the decoding is omitted and speed-up of processing has been achieved by using the temporarily stored decoded data when the decoded data is again needed for redrawing or the like.
FIG. 9 shows a block diagram of a conventional data managing device. Coded data storing means stores coded data. The coded data is decoded by decoding means, and is stored in decoded data storing means. Access order storing means stores the order of access among decoded data. Priority order managing means determines the priority of each piece of decoded data according to the order of access. And when a memory area is insufficient, a piece of decoded data with low priority is deallocated from the memory area by an LRU algorithm.
However, since this conventional data managing device collectively stores and deallocated the whole of the decoded data obtained by decoding the coded data, the whole of the decoded data is held stored even when part of the decoded data is only frequently used, and the use efficiency of the memory area is lowered.
As an example, take a drawing processing wherein both a background picture and a picture 1 which are compressed bitmaps overlap each other and the picture 1 is replaced with a picture 2 and then a picture 3 as shown in FIG. 10 which is a view for explaining the problem of the conventional data managing device.
When the picture 1, the picture 2 and the picture 3 include a transparent color, since it is necessary that the background picture be seen through the part of the transparent color, it is necessary to redraw the part of the background picture where the picture 1 is replaced with another picture before drawing the replacing picture.
In this case, when the conventional data managing device is used, the decoded data of the background picture that is accessed every time the picture is replaced has higher priority than the picture 1, the picture 2 and the picture 3. In a case that the memory area is insufficient, if the decoded data of the background picture is stored, the decoded data of another picture is not able to be stored. Therefore it is necessary to re-decode the coded data of the picture 1, the picture 2 or the picture 3 every time the picture is replaced and so that processing requires time.